BECKER, Tracy A; RODNICK, Kenneth J*; Idaho state univ: Evidence that hypoxia stimulates lactate efflux but not glucose transport in cardiac tissue of rainbow trout (Oncorhynchus mykiss)

Glucose transport is the first step of glucose metabolism. Although hypoxia is a potent stimulus of glucose transport in mammalian cardiac and skeletal muscle, little is known about the effects of reduced oxygen on teleost cardiac energy metabolism and performance. We therefore compared the effect of hypoxia and normoxia on glucose transport and lactate efflux in isolated, non-contracting ventricular tissue from rainbow trout, a hypoxia-intolerant species. Ventricles were rapidly excised, cut into uniform strips, incubated in either oxygenated (99.5% O₂, 0.5% CO₂, PO₂ = 640 mm Hg) or hypoxic (99.5% N₂, 0.5% CO₂, PO₂ = 20 mm Hg) trout Ringers supplemented with 5 mM glucose for 60 min, transferred to glucose free Ringers for 10 min, then to the transport medium (5 mM ³H-2-deoxyglucose, 1.0 µCi/mL; ¹⁴C-mannitol, 0.1 µCi/mL) for 20 min. Tissues were removed, and processed for liquid scintillation counting; glucose transport was calculated as nmoles/mL intracellular water/20 min. Lactate was measured in the buffer recovered from the final incubation. Hypoxia increased lactate efflux from ventricle tissue 5-fold compared to oxygenated conditions (2.8 � 0.3 vs. 14.7 � 1.3 µmol/g/60 min; P < 0.001) while there were no differences in glucose transport. However, cytochalasian B decreased glucose transport by ~ 60% under both conditions (P < 0.001), but did not affect lactate efflux. Our results demonstrate that hypoxia promotes glycolysis in the rainbow trout similar to mammals. However, unlike mammals, hypoxia did not stimulate glucose transport. Therefore, stored glycogen may be the primary substrate for increasing anaerobic glycolysis in the trout cardiomyocyte. Funded by NIH P20RR16454